1. Field of the Invention
The present invention relates to devices and driving methods thereof. Specifically, the present invention relates to semiconductor devices and driving methods thereof. More specifically, the present invention relates to liquid crystal display devices and driving methods thereof, and particularly, liquid crystal display devices with improved contrast and driving methods thereof, and methods for improving quality of moving images.
2. Description of the Related Art
In recent years, there has been growing interests in thin display devices. As substitutes for CRT displays, liquid crystal displays, plasma displays, projection displays, and the like have been developed and becoming popular. Further, field emission displays, inorganic electroluminescence displays, organic electroluminescence displays, electronic paper, and the like have been developed as next-generation display devices.
In a display portion provided in the above-described display device, pixels which are minimum units for forming an image are arranged. Each of the pixels emits light with luminance in accordance with image data, and an image is displayed on the display portion.
When a moving image is displayed by such a display device, a plurality of images are displayed quickly, several tens of times per second. A cycle in which a plurality of images are displayed (or a cycle in which image data is input to the display device) is referred to as one frame period.
Here, driving methods of display devices can be classified according to temporal distribution of luminance of a pixel in one frame period. In a hold-type driving method which is used mainly in an active matrix display device, luminance of a pixel is constant in one frame period. On the other hand, in an impulse-type driving method which is used in CRT and the like, intense light is emitted once in one frame period and thereafter, luminance of a pixel immediately decreases and light is not emitted any more. In the impulse driving method, a non-lighting state dominates the most part of one frame period.
It has become obvious that hold-type display devices have the following problems of motion blur: in displaying moving images, a moving object appears to have traces when a part of the image moves or the whole image is blurred when the whole image moves. It is said that this is because an image displayed on a hold-type display device is kept still during one frame period, while a human predicts motion of the moving object and sees a direction in which the moving object is supposed to move. That is, this is because a discrepancy between movement of human eyes and movement of the moving image. In an impulse-type display device, since an image appears for a moment and disappears immediately, there is no problem of such a discrepancy between human eyes and a moving image, and thus, a problem of motion blur does not occur.
Typically, two methods for solving the problem of motion blur in the hold-type display device are proposed (see Patent Document 1: Japanese Published Patent Application No. H4-302289). The first method is a method in which a period for displaying an original image and a period for display a black image are provided in one frame period. By this method, display can be closer to pseudo impulse-type display, and fewer afterimages can be perceived; thus, quality of moving images can be improved (see Patent Document 2: Japanese Published Patent Application No. H9-325715; and Patent Document 3: Japanese Published Patent Application No. 2000-200063). The second method is a method in which one frame period is shortened (i.e., frame frequency is increased). Accordingly, movement of an image can follow movement of human eyes and movement of the image is smooth, which leads to improvement of quality of moving images (see Patent Document 4: Japanese Published Patent Application No. 2005-268912). Further, as a technique for improving the first method, a method in which an image which is darker than an original image is displayed instead of a black image to improve quality of moving images is disclosed (see Patent Document 5: Japanese Published Patent Application No. 2002-23707; Patent Document 6: Japanese Published Patent Application No. 2004-240317; Non-patent Document 1: “New Technologies for Large-Sized High-Quality LCD TV”, SID'05 DIGEST, 60.2, pp. 1734-1737 (2005); Non-patent Document 2: “Amorphous Silicon Based 40″ LCD TV Using Ultra Fast OCB Mode”, SID'06 DIGEST, 69.4, pp. 1950-1953 (2006); and Non-patent Document 3: “Progress of IPS-Pro Technology for LCD-TVs”, SID'06 DIGEST, 69.5, pp. 1954-1957 (2006)). Further, a method in which a driving method is changed depending on conditions is disclosed (see Patent Document 7: Japanese Published Patent Application No. 2002-91400; and Patent Document 8: Japanese Published Patent Application No. 2004-177575).
In addition, a contrast ratio is one of indicators of quality of images to be displayed by a display device. The contrast ratio refers to a ratio of contrast of luminance that the display device can express (a ratio between the maximum luminance and the minimum luminance). A display device with higher contrast ratio can display a sharper and clearer image whose dynamic range of brightness is larger. Methods for improving a contrast ratio of a display device using a backlight are disclosed in Patent Document 9 and Non-patent Document 4, for example (Patent Document 9: Japanese Published Patent Application No. 2007-25187; and Non-patent Document 4: “RGB-LED Backlights for LCD-TVs with 0D, 1D, and 2D Adaptive Dimming”, SID '06 DIGEST, 44.4, pp. 1520-1523 (2006)).